WO2016075646A1

WO2016075646A1 - A gasifier stove

Info

Publication number: WO2016075646A1
Application number: PCT/IB2015/058740
Authority: WO
Inventors: David Roydon LELLO; William Albert Hadlow
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-11-12
Filing date: 2015-11-12
Publication date: 2016-05-19
Anticipated expiration: 2017-05-12

Abstract

A forced airflow gasifier stove (100) for indoor use is provided which includes a combustion chamber (144) mounted within a housing (102) which cooperates with a blower (108, 110) for directing air upwards (118) through the housing (102). The combustion chamber (144) has a primary air inlet zone (166) near an operatively lower end thereof (164) and an open upper end (168). A cap assembly (140) connects to the open upper end of the combustion chamber (142) and has a plurality of apertures or slots (156) therein which, in an assembled condition of the forced airflow gasifier stove (100), are partially occluded by moving the combustion chamber (144) relative to the cap assembly (140) so as to form an adjustable secondary air inlet zone (176, 156) near the upper end (142) of the combustion chamber (144).

Description

A GASIFIER STOVE

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to South African provisional patent application number 2014/08267 filed on 12 November 2014 and South African provisional patent application number 2015/05412 filed on 28 July 2015, both of which are incorporated by reference herein. FIELD OF THE INVENTION

This invention relates to a gasifier stove. In particular, it relates to a forced airflow gasifier stove suitable for use indoors. BACKGROUND TO THE INVENTION

Traditionally, biomass such as wood and charcoal have been used as the primary source of fuel for cooking and heating in households with limited or no access to electricity. Most existing stoves or cookers do not cause complete combustion of volatile organic compounds, and the resultant smoke contains soot and harmful gases such as carbon monoxide. Not only are these gases harmful to the environment, but they pose a health risk to persons residing in confined living quarters. Open fires and conventional stoves may also pose a substantial fire risk.

Forced air convection stoves have been developed in which a blower directs air towards a combustion chamber. Some of these stoves, for example the stove described in US patent publication number US 2009/0165769 and PCT publication WO 2007/036720, have a primary air inlet at a lower end of the combustion chamber and a secondary air inlet at an upper end of the combustion chamber to result in more complete and effective burning of the fuel. One problem associated with these forced air convection stoves is that the secondary air inlet around the periphery of the combustion chamber results in the hottest part of a resultant flame to be concentrated in the centre of the combustion chamber rather than being evenly spread across the combustion chamber. A further problem is that different stoves must be provided for different biofuels, as the power output of the stoves cannot be easily varied. Embodiments of the invention aim to address these and other problems, at least to some extent.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a forced airflow gasifier stove, comprising a combustion chamber mounted within a housing which cooperates with a blower for directing air upwards through the housing, the combustion chamber having a primary air inlet zone near an operatively lower end thereof and an open upper end, wherein a cap assembly connects to the open upper end of the combustion chamber, the cap assembly having a plurality of apertures or slots therein which, in an assembled configuration of the stove, are partially occluded by moving the combustion chamber relative to the cap assembly so as to form an adjustable secondary air inlet zone near the upper end of the combustion chamber.

Further features provide for the cap assembly to include a cover that that fits onto an open upper end of the housing so as to define a compartment between the housing and the combustion chamber; for the cover to have an operatively downward projecting flange which has the plurality of apertures or slots therein; and for the flange to cooperate with the open upper end of the combustion chamber either directly or by means of an adapter.

A still further feature provide for where the stove to be provided with a number of different adapters which each fit between the flange of the cap assembly and the open upper end of the combustion chamber, the adapters enabling combustion chambers of different diameters to be connected to the cap assembly.

In one embodiment, the flange has a plurality of slots therein that form a comb having a plurality of teeth, wherein the teeth fit at least partially inside the open upper end of the combustion chamber or at least partially inside the adapter. The cap assembly is moveable in an axial direction relative to the combustion chamber to adjust the size of the partially occluded apertures or slots, thereby controlling the ratio of air flowing through the primary air inlet zone and air flowing through the secondary air inlet zone.

Yet further features provide for the stove to include a conduit which extends inside the combustion chamber from bottom to top, the conduit including apertures near an operatively lower end thereof to form the primary air inlet zone near the operatively lower end of the combustion chamber, and apertures at an operatively upper end thereof to form a tertiary air inlet near the operatively upper end of the combustion chamber. Further features provide for the housing to contain a heat shield which surrounds the combustion chamber and is spaced therefrom, and which cooperates with a second operatively downwardly projecting flange formed as part of the cap assembly to define a further compartment between the heat shield and the combustion chamber, the heat shield being provided with a mounting formation for receiving and holding the combustion chamber in a central position relative to the heat shield and the housing.

The housing and the combustion chamber may both be repurposed containers, such as cylindrical cans.

Still further features provide for the blower to be a fan housed in a separate base component onto which the housing is placed in use, the base component having an air inlet on a side wall and an air outlet which aligns with an air inlet of the housing. The fan may be powered by a power source and have a controller for adjusting the speed of the fan.

Yet further features provide for a frame to be connected to the base component, the frame to extend upwards to provide a cooking support surface above the combustion chamber. The housing may include a single vertically fitted handle to permit the housing to be lifted with one hand.

The stove is preferably configured to operate as a top lit up draft (TLUD) stove, in which fuel housed in the combustion chamber burns from top to bottom in use and wherein the fuel is biomass, preferably wood pellets. The above and other features of the invention will be more fully understood from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Figure 1 A is a perspective view of a first embodiment of a forced airflow gasifier stove according to the invention, including a frame that provides a cooking support surface;

Figure 1 B is a top plan view of the stove and frame of Figure 1 A;

Figure 1 C is a perspective view of the frame without the stove;

Figure 2 shows only the stove of Figure 1 A; Figure 3 is an exploded view of the components of the stove of Figure 2;

Figure 4 is a top plan view of the housing of the stove with its other components removed; Figure 5 is a perspective view of a cap assembly of the stove of Figure 2;

Figure 6 is a side elevation of the cap assembly and combustion chamber of the stove of

Figure 2;

Figure 7 A is side elevation of the cap assembly connected to an adapter which fits a narrower combustion chamber;

Figure 7B is a side elevation of the cap assembly connected to an adapter which fits a wider combustion chamber;

Figure 8A is a top plan view of a heat shield of the stove of Figure 2 which includes a mounting formation for the combustion chamber;

Figure 8B is bottom plan view of the heat shield in which the combustion chamber and central conduit has been fitted;

Figure 9A is a perspective view of the combustion chamber with its central conduit;

Figure 9B shows only the central conduit;

Figure 10 is a sectional elevation which shows airflow through the stove;

Figure 1 1 is a perspective view of an alternative embodiment of a cap assembly;

Figure 12A is a perspective view of a second embodiment of a forced airflow gasifier stove according to the invention;

Figure 12B is a top plan view of Figure 12A:

Figure 13 is a top plan view of the interior of the housing of the stove of Figure 12A, containing a heat shield, combustion chamber and central conduit; and

Figure 14 is a bottom perspective view of the combustion chamber of the stove of Figure

12A.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Figures 1 to 10 show a first embodiment of a forced airflow gasifier stove (100). The stove (100) has a housing (102) which, in this embodiment, is a repurposed 5 litre paint can. A vertically extending handle (104) is attached to the housing, in this illustration by means of screws (106), which permits the stove to be lifted with one hand and carried.

The stove (100) cooperates with a blower (108) which directs air upwards through the housing (102). The blower includes a fan (1 10) mounted within a separate base component (1 12) onto which the housing is placed in use. The fan preferably produces an air flow of about 12 cfm (5.7 x 10-3 m³/s) and may be a 5 V or 12 V fan. The base component (1 12) has a squat substantially square profile with an air inlet (1 14) on a side wall (1 16) and an air outlet (1 18) on a top surface (1 16), shown most clearly in Figure 1 C. The air outlet (1 14) aligns with an air inlet (122) of the housing, shown most clearly in Figure 10, when the housing is placed on top of the base component (1 12) as shown in Figure 1 A.

The base component (1 12) may have a power source such as a battery or may have an input port (not shown) through which power can be supplied from an external power source such as an AC/DC adapter, rechargeable battery or photovoltaic cell, and has an on/off switch (124). The base component has the dual purpose of providing forced airflow by means of the blower and of providing an insulation layer between the stove and the surface on which the stove is placed in use, so as to avoid damaging the surface or causing a fire. The base component may also have a controller, such as a variable potentiometer, for adjusting the speed of the fan. It will be appreciated by those skilled in the art that the base component may alternatively be provided with pulsed width modulation (PWM) speed control. The top surface of the base component (1 12) is provided with an insulation ring (126) made of heat insulating material onto which the stove is placed.

The air inlet (122) of the housing is shown in Figures 4 and 10 and includes a central hole (127) covered by a raised circular plate (129) mounted on pins (131 ).

A frame (128) is connected to the base component and, as shown most clearly in Figure 1 C, comprises two U-shaped bent metal rods (130) received within locating formations (132) provided near the corners of the base component (1 12) on its top surface (1 16). The metal rods (130) support a stove top (134) which, as mostly clearly shown in Figure 1 A, includes a square frame (136) with four support arms (138) extending from the corners of the frame towards a centre thereof to define a horizontal cooking support surface above the stove. The four support arms (138) are metal plates that are welded to the metal square frame (136). In use, a cooking implement such as a pot, pan, grill or the like is supported on top of the four support arms (138) of the stove top (134).

The components of the stove (100) are shown in more detail in the exploded view of Figure 3. The housing includes a cap assembly (140) which connects to an upper end (142) of a combustion chamber (144) and holds the combustion chamber (144) centrally within the housing. In this embodiment, the combustion chamber (144) is a repurposed 100 mm cylindrical tin food can. The cap assembly (140) is shown in detail in Figure 5. In this embodiment, the cap assembly is made of metal sheets that have been machined to the desired size and dimensions and bent into shape. In other embodiments, the cap assembly may be punched or cast. The cap assembly includes an annular-shaped cover (148) that has a central hole (149) and which fits onto an open, upper end (150) of the housing so as to define a compartment (152) between the housing and the combustion chamber as shown in Figure 10. The cover (148) of the cap assembly has three clips (151 ) along its periphery which enable it to clip over the upper end of the housing and has an operatively downwardly projecting flange (154) which has a plurality of slots (156) therein. The plurality of slots (156) form a comb with a plurality of teeth (158), wherein the teeth (158) fit at least partially inside the open upper end (142) of the combustion chamber (144) so that the cap assembly connects to the open upper end of the combustion chamber in a way that partially occludes the slots (156). The cap assembly (140) is movable in an axial direction relative to the combustion chamber (144) to adjust the size of the partially occluded slots (156). The partially occluded slots form an adjustable secondary air inlet zone (160) near the upper end of the combustion chamber, as most clearly shown in Figure 6 and as will be described below. A conduit (162) extends inside the combustion chamber from bottom to top, and has a first set of apertures (164) near an operatively lower end thereof to form a primary air inlet zone (166) near the operatively lower end of the combustion chamber. The conduit (162) also includes a second set of apertures (168) at an operatively upper end thereof to form a tertiary air inlet zone (170) near the operatively upper end of the combustion chamber (144). The conduit is preferably made of a heat resistant material such as stainless steel. The conduit has an open operatively lower end which is surrounded by a substantially hemispherical base portion (145). In use, the hemispherical base portion of the central conduit provides the floor of the combustion chamber onto which fuel is loaded. An operatively upper end (146) of the central conduit (162) is closed and fans out into a flute-like shape. In use, the lower end receives air from the blower.

A heat shield (172) fits within the housing (102) and surrounds the combustion chamber (105) while being spaced therefrom. The heat shield is cylindrical in shape and made from a rectangular sheet of metal that has been rolled into shape and two opposite ends joined with pop rivets (173). The heat shield has a larger diameter than the combustion chamber, but a smaller diameter than the housing. In the assembled condition of the stove, the heat shield is contained within the housing and it substantially surrounds the combustion chamber. The heat shield (172) cooperates with a second operatively downwardly projecting flange (174) on the cap assembly (140) to define a further compartment (176) between the heat shield (160) and the combustion chamber (144). The second downward projecting flange (174) slides over the heat shield in a mechanical fit with the heat shield. When the second downward projecting flange and heat shield are fitted together, they define the further compartment (176) within the housing for the flow of air from the blower between the heat shield and the combustion chamber, as shown in Figure 10. As shown most clearly in Figures 8A and 8B, the heat shield is provided with a mounting formation (178) for receiving and holding the combustion chamber in a central position relative to the heat shield and the housing. The mounting formation includes a ring (180) with four support formations (182) in the form of metal plates. The ring is attached to the heat shield by suitable connectors (184), in this embodiment bent pieces of metal attached to the heat shield (172) and the ring (178).

Figure 8B is a bottom view showing the bottom of the combustion chamber (144), where the bottom of the combustion chamber is formed by the hemispherical base portion (145) of the central conduit (162). An air inlet hole (186) of the central conduit is also shown in this view. The ring (180) is sized to be only slightly larger than the combustion chamber, so that an operatively lower end of the combustion chamber can be fitted within the ring.

While the flange may connect directly to the open upper end of the combustion chamber, Figure 7A and 7B shows an alternative arrangement in which two adapters (190, 192) is provided which each fit between the flange (154) of the cap assembly (140) and the open upper end of the combustion chamber (144). The adapters enable combustion chambers of different diameters to be connected to the cap assembly, for example a narrower diameter combustion chamber shown in Figure 7A and a wider diameter combustion chamber shown in Figure 7B. The adapters may fit onto the flange by means of a press fit and may also be a press fit with the combustion chamber, or may have a screw thread that cooperates with the combustion chamber or other attachment means.

The operation of the stove (100) will now be described with reference to Figure 10, in which arrows indicate the flow of air through the stove. Firstly, the stove is filled with suitable biomass material such as wood pellets (194). The wood pellets proposed to be used are highly efficient as the wood has been compressed to a point where it is 50% more dense than natural wood and dried such that it has only 6-8% moisture. It has a density of 650 g/dm³ and energy of approximately 20 MJ/kg. The wood pellets produce a slow and steady combustion at temperatures higher than raw wood. To aid the combustion of the wood pellets in initial stages of igniting it, a small amount (approximately 50 ml) of a flammable liquid such as turpentine may be sprinkled on the top layer of pellets. After approximately 30 seconds the turpentine is absorbed by the wood pellets and they may be lit with long safety matches or a gas fire lighter. When lit, the stove may initially only produce an orange flame. After lighting the fire, the fan may be turned on. The fan should ideally be on low speed until the bottom flame is hot and pyrolysis is occurring. As the wood chips start burning, an upper flaming pyrolysis front is formed which burns from top to bottom. The stove therefore operates as a top lit up draft (TLUD) stove, in which fuel housed in the combustion chamber burns from top to bottom in use. Air is directed by the blower (108) upwards towards the housing and the air enters the central hole (127) in the housing and flows into the further compartment (176). The air then enters the conduit (162) as well as moving upwards alongside the combustion chamber. The air inside the conduit enters the combustion chamber (144) through the first set of apertures (164) at the primary air inlet zone.

Hot air that has travelled through the compartment (176) also enters the combustion chamber at its operatively upper end through the partially occluded slots (156) at the secondary air inlet zone. Furthermore, hot air that rises through the conduit enters the combustion chamber through the second set of apertures (168) at the top of the conduit at the tertiary air inlet zone.

The TLUD stove uses limited oxygen from the primary air inlet zone to pyrolyse the wood pellets and release volatile components such as combustible gases and tars from the wood, collectively referred to as wood gases. The forced airflow assists in the release of the wood gases from the wood pellets. The wood gases, a syngas fuel, is directed upwards by the fan where it mixes with the hot air in the housing that enters the combustion chamber through the secondary air inlet zone and tertiary air inlet zone. The hot air and gases are mixed according to the pressure of the air entering the combustion chamber. Combustion of the wood gases produces a hot and substantially smokeless flame. The size of the partially occluded slots that form the secondary air inlet zone determine the ratio of the mixture of hot air and gases. By adjusting the axial position of the cap assembly to change the size of the slots, the ratio of hot air to wood gases can be adjusted from between approximately 3:1 to 6:1 .

It was found that the inclusion of the central conduit, which delivers air to central zones within the combustion chamber enhances the efficiency of the combustion process thereby ensuring a stable flaming pyrolysis front and constant flame. In particular, by having the secondary and tertiary air inlet zones close together but in opposing directions, it was found the resultant flame above the stove was more evenly spread out and was not centrally concentrated. The opposing air inlet zones also results in the upwards motion of hot air to be slowed, thereby enabling more complete combustion. The combustion chamber is in the middle of the housing and although it reaches temperatures of as high as 700 °C, it is insulated by the housing making the stove safe to use indoors. The inclusion of a heat shield between the combustion chamber and the housing further insulates the combustion chamber.

In one experiment, the stove used approximately 500 g of pellets per hour. The residual weight of the char was approximately 100 g, thus -80% of the mass was converted into energy, making the stove exceptionally energy efficient. The heat content of the gas was -18 kJ/g, so consumption of -4-10 g/min produces approximately 1 .2-3.0 kW. The burn rate is controlled by the speed of the fan. The fan improves the efficiency and control of the combustion by approximately 40-60%, whilst the fan consumes only approximately 1 .2 W. Thus the power used to induce a draft is insignificant considering the control of combustion that is achieved. The control enables a design of a stove that is more compact than a natural draft stove. The forced airflow stove is also more manageable in windy conditions. The forced airflow gasifier stove produces low amounts of harmful carbon monoxide (CO) and particle matter. The combustion of wood gases produces only carbon dioxide, water and heat.

The stove is easy to use and operates with minimal attention for up to an hour at a time. As the stove is a batch load device, a decision needs to be made on how much fuel to load prior to lighting. Fuel cannot be added at a later stage during use of the stove and the only way to acquire heat for a longer time is to empty the stove and to reload it again. Although the combustion chamber can be filled with 500 g of pellets, smaller amounts such as, for example, 150 g, can be loaded at a time.

After approximately 5 minutes of the wood pellets being alight, pyrolysis of the wood pellets is expected to occur resulting in the release of wood gases within the combustion chamber. The wood gases mix with heated air from the secondary and tertiary air inlet zone to produce a substantially smokeless flame at the open upper end of the combustion chamber that is usually blue and orange in colour. Once the smokeless flame is produced, a user has approximately 10 minutes of cooking or heating time per 100 g of pellets loaded. The time can vary based on the speed of the fan and the size of the secondary air inlets amongst other factors. The flame or heat produced can be set within a range from a low, medium or high setting according to the needs of the user by adjusting the speed of the fan and/or the size of the partially occluded slots in the secondary air inlet zone.

The stove is modular and various parts can be replaced or interchanged. In particular, by using various adapters (190, 192) that are supplied together with the stove, differently sized combustion chambers can be fitted within the same stove without requiring any other modifications. Differently sized combustion chambers may be desired for different heating applications or for burning of different forms of fuel. Modularity simplifies manufacture but also improves the serviceability and repair of the stove in the field. For example, the electrical components are all in the base component that houses the fan. The modular nature of the base component is an especially useful feature of the invention, as it may find use in other applications. A variety of additional components may be added to the base component such as a UV/LED light and reversible fan so as to create an insect trap. It may be provided with a chamber above the base component having a heating element and thermostat for use as a food drier or yoghurt maker. Alternatively the base component may be provided with additional cooling fins and a water evaporator for use as a liquids chiller. The stand or part of the stand may be fitted with an LED light to create a desktop lamp.

The embodiment of the forced airflow gasifier stove described thus far has an all metal design that is partly recycled and relatively lightweight, yet durable. The aim of the design of the stove is to provide a stove that is easy to operate and maintain and that is affordable to buy. It will be appreciated by those skilled in the art that the housing and combustion chamber need not be made of recycled cans and that various parts of the stove may be manufactured from different materials without departing from the scope of the invention.

It is envisaged that the stove is fuelled inexpensively with a renewable fuel made from wood waste products that costs less than, for example, paraffin. It will be appreciated by a person skilled in the art, however, that any appropriate fuel may be used with the stove without departing from the scope of the invention. The size of the partially occluded slots and thus the secondary air intake can be varied according the type of fuel that is combusted in the chamber based on the amounts of volatile and combustible gases that is derived from the fuel. It will be appreciated by one skilled in the art that any suitable adjustment mechanism capable of moving the cap assembly or the part of the cap assembly within the open end of the combustion chamber in an axial direction relative to the combustion chamber may be incorporated in the stove to obtain an adjustable secondary air inlet zone. It is foreseen that the adjustment mechanism may also be provided with an external controller which is operable by a user while the stove is in operation. In this manner the adjustment mechanism is configured to move the cap assembly or the comb thereof relative to the combustion chamber so as to vary the size of the partially occluded slots and the volume of secondary air intake while the stove is in operation.

The above description is by way of example only and it should be appreciated that numerous changes and modifications may be made to the forced airflow gasifier stove without departing from the scope of the invention. For example the number of air inlets and air outlets may be altered according to changes in the shape, size and capacity of the stove. Any suitable fan that fits into a base component may form part of the stove and it may be powered by any suitable or available power source.

Figure 1 1 , for example, shows an alternative cap assembly (200) that is moulded rather than being made from sheet metal. As such a cap assembly can be moulded to a high degree of tolerance, the comb (202) that forms the flange may provide a sufficient press fit with the combustion chamber that it not necessary to have any bent teeth or other fastening means.

Figures 12 to 16 show a second embodiment of a forced airflow gasifier stove (300) of larger construction, in this case having a housing (302) made from a repurposed 20 litre paint can. A base component housing a fan is not shown in these views. The operation of the stove (300) is similar to the stove (100) of the first embodiment, but in this embodiment a combustion chamber (304) includes a gauze mesh (306) which supports solid fuel in a spaced manner above lower air inlets of a central conduit (308), so that the fuel does not constrict the flow of air from the apertures into the much wider combustion chamber.

Throughout the specification and claims unless the contents requires otherwise the word 'comprise' or variations such as 'comprises' or 'comprising' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

CLAIMS:

1 . A forced airflow gasifier stove, comprising a combustion chamber mounted within a housing which cooperates with a blower for directing air upwards through the housing, the combustion chamber having a primary air inlet zone near an operatively lower end thereof and an open upper end, wherein a cap assembly connects to the open upper end of the combustion chamber, the cap assembly having a plurality of apertures or slots therein which, in an assembled configuration of the stove, are partially occluded by moving the combustion chamber relative to the cap assembly so as to form an adjustable secondary air inlet zone near the upper end of the combustion chamber.

2. A stove as claimed in claim 1 , wherein the cap assembly includes a cover that fits onto an open upper end of the housing so as to define a compartment between the housing and the combustion chamber and has an operatively downward projecting flange which has the plurality of apertures or slots therein, the flange cooperating with the open upper end of the combustion chamber either directly or by means of an adapter.

3. A stove as claimed in claim 2, where the stove is provided with a number of different adapters which each fit between the flange of the cap assembly and the open upper end of the combustion chamber, the adapters enabling combustion chambers of different diameters to be connected to the cap assembly.

4. A stove as claimed in claim 2 or claim 3, wherein the flange has a plurality of slots therein that form a comb having a plurality of teeth, wherein the teeth fit at least partially inside the open upper end of the combustion chamber or at least partially inside the adapter.

5. A stove as claimed in any one of claims 1 to 4, wherein the cap assembly is moveable in an axial direction relative to the combustion chamber to adjust the size of the partially occluded apertures or slots, thereby controlling the ratio of air flowing through the primary air inlet zone and air flowing through the secondary air inlet zone.

6. A stove as claimed in any one of the preceding claims, which includes a conduit which extends inside the combustion chamber from bottom to top, the conduit including apertures near an operatively lower end thereof to form the primary air inlet zone near the operatively lower end of the combustion chamber, and apertures at an operatively upper end thereof to form a tertiary air inlet near the operatively upper end of the combustion chamber.

7. The stove as claimed in any one of the preceding claims, wherein the housing contains a heat shield which surrounds the combustion chamber and is spaced therefrom, and which cooperates with a second operatively downwardly projecting flange formed as part of the cap assembly to define a further compartment between the heat shield and the combustion chamber, the heat shield being provided with a mounting formation for receiving and holding the combustion chamber in a central position relative to the heat shield and the housing.